1. Field of the Invention
This invention relates to a heat-sensitive recording material which is superior in dispersibility, image contrast, oil resistance, solvent resistance, and optical readability in the near infrared region.
2. Prior Art
A heat-sensitive recording sheet that utilizes a heat-color-forming reaction occuring between a colorless or pale-colored chromogenic dyestuff and a phenolic material, or an organic acid is disclosed, for example, in the Japanese Patent Publication Nos. 4160/1968 and 14039/1970 and in the Japanese Laid-Open Patent Publication No. 27736/1973, and is now widely applied for practical use.
In general, a heat-sensitive recording sheet is produced by applying on a support, such as paper, film etc., the coating which is prepared by individually grinding and dispersing a colorless chromogenic dyestuff and a color-developing material into fine particles, mixing the resultant dispersion with each other and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. The coating, when heated, undergoes instantaneously a chemical reaction which forms a color. In this case various bright colors may be formed depending upon the selection of colorless chromogenic dyestuff.
These heat-sensitive recording sheets have now been finding a wide range of applications, including medical or industrial measurement recording instruments, terminal printers of computer and information communication systems, facsimile equipments, printers of electronic calculators, automatic ticket vending machines, and so on.
In recent years, as the heat-sensitive recording systems are widely used and the applications of such recording are diversified, high image density is now required for the improvement of the resolution. The heat energy of the thermal head in the recording equipments capable of such high density is more minimized. Therefore, it is required that the heat-sensitive recording sheet has a higher color-forming sensitivity sufficient for producing clear chromogenic record with such small heat energy.
Meanwhile, the heat-sensitive recording sheets are inevitably touched with the hand of man, in view of their function as recording sheets of the information.
As the fingers of the operator are often adhered by solvents such as alcohols, acetones, etc., or by oily substances such as his hair tonic daily used and sebum, it may be said that the heat-sensitive recording sheets are most frequently contaminated by such substances. In general, the heat-sensitive recording sheets have insufficient stability against these oily substances and the solvents such as alcohols, acetones, etc., so that the density of the developed color image on the contaminated part is often reduced or disappeared. The contaminated white ground causes the phenomenon of discoloration or color forming. Their reasons can not be sufficiently elucidated yet, but it is supposed that such substances partly dissolve the coloring layer consisting of the fine granular basic colorless dyestuff and organic developer or coloring reactant thereof, or make the coloring layer or coloring reactant thereof instable.
Also, the developed images disappear, or the color reaction through the solvent between dyestuff and color-developing agent, i.e. the color development of the ground color, occurs.
In order to improve the oil resistance and solvent resistance, there have been proposed either a color-developing layer comprising a leuco dyestuff, an organic color-developing agent and organic acid metal salt; or a protective layer on a color-developing layer. The proposed methods do not yet provide a sufficient result.
Further, these heat sensitive recording sheets comprising the combination of a leuco-dyestuff and a color-developing agent are utilized as thermosensitive labels in POS-system. However, since the color formation is in the visible region, these recording sheets cannot be adapted for reading by a semi-conductor laser in the near infrared region which is used as a bar code scanner.
Besides the heat-sensitive color-developing system in which the above colorless leuco dyestuff is used, a chelate type color-developing system under the use of metal compounds is known.
For examples, the Japanese Patent Publication No. 8781/1957 describes the combined use of iron stearate (electron acceptor) with tannic acid or gallic acid, and the Japanese Patent Publication No. 6485/1959 describes the combined use of an electron acceptor such as silver stearate, iron stearate, gold stearate, copper stearate or mercury behenate with an electron donor such as methyl gallate, ethyl gallate, propyl gallate, butyl gallate or dodecyl gallate. Since these heat-sensitive system by means of the heat energy of light, they bring the troubles of accumulated residues and sticking under applying to heat-sensitive recording system which uses the thermal printing heads. In this case, they have as disadvantages a low image density, greenish color, poor brightness of the background, inferior stabiling against solvents such as alcohols, and the flowing-out of the color-developing layer. Further, Japanese Laid-Open Patent Application No. 89193/1984 describes a combination of a color-developing system using a leuco dyestuff and a color-developing agent and of a color-developing system using a metal compound of higher fatty acid ferric salt and polyvalent phenol. However, such combination is disadvantageous in costs, since it requires a protecting layer for hiding colored parts. Further, it has a defect that solvents such as alcohols are penetrated through the pin holes of a protecting layer, resulting in coloring (contamination) caused by a reaction between a leuco dyestuff and a color-developing agent which are present in a color-developing layer.
The inventors described in Japanese Laid-Open Patent Application No. 11681/1987 (Japanese Patent Application No. 150090/1985) that there was provided a heat-sensitive recording material which is suitable for a high speed recording and has a high stability of developed image against oily substances and solvents such as alcohols, by using the combination of a phenolic derivative and a higher fatty acid iron salt having 16-35 carbon atoms.
However, the higher fatty acid iron salt having 16-35 carbon atoms has the defects that fine foams are produced, the viscosity is increased and the coagulates are formed, in grinding to a particle size of several microns or smaller by means of a ball mill, attritor, sand grinder, etc.
Further, the above higher fatty acid iron salt itself has a poled yellow color, and hence the background of the heat-sensitive recording material obtained by applying the dispersed coating material is colored, which brings the defect of providing an inferior image contrast.